I built my kiddo a hand plane, but it really is just a full sized hand plane that I replaced the blade with another piece of wood. Maybe just buy an el cheapo wood plane off ebay like this one and swap out the blade?

I was hoping to find one for my 3 year old. He uses blocks or any shape thing he can find to plane things. I looked up Little Tykes and bid on a couple but did not see any planes. I think I might take my #3 and take the blade out. Good ideas. Thanks.